An OLED device could be fabricated from small molecule or polymeric materials. A typical device structure of a polymer light-emitting diode (PLED) consists of an anode (e.g. indium-tin-oxide (ITO)), a hole injection layer (e.g. poly(3,4-ethylenedioxythiopene):poly(styrene sulfonic acid) (PEDOT:PSS), or polyaniline), an electroluminescent layer, and a cathode layer (e.g. barium covered with aluminum). Among the two organic layers, the function of the hole injection layer is to provide efficient hole injection into subsequent adjacent layers. In addition, hole injection layer also acts as a buffer layer to smooth the surface of the anode and to provide a better adhesion for the subsequent layer. The function of the electroluminescent layer is to transport both types of carriers and to efficiently produce light of desirable wavelength from electron-hole pair (exciton) recombination. Solution-processed polymer light-emitting diodes (PLEDs) have demonstrated advantages such as ease of fabrication, low fabrication cost, and the possibility of producing large area displays. Hole injection layers used in typical PLEDs is usually a conductive polymer doped with polymeric acid in the form of aqueous particle suspension, e.g., PEDOT/PSS. Although this works for some applications, it still has some drawbacks: (1) The particle nature usually requires a relatively thicker layer to avoid pixel short and strong leakage current. (2) The polymeric acid dopant is usually the major component to achieve the required conductivity. This makes the formed layer non-homogeneous in bulk and surface composition. (3) The dopant acid group is suspected to be electrochemically unstable during device operation.